Update gwpy dependency to a specific hash to catch up with matplotlib change
The detchar test was failing with
ax = <Axes: xlabel='Time [$\\mathrm{s}$]', ylabel='Frequency [$\\mathrm{Hz}$]'>
def _get_axes_class(ax):
if isinstance(ax, SubplotBase):
> return ax._axes_class
E AttributeError: 'Axes' object has no attribute '_axes_class'
/usr/local/lib/python3.8/site-packages/gwpy/plot/colorbar.py:141: AttributeError
due to an update in matplotlib.
This gwpy commit (https://github.com/gwpy/gwpy/commit/76b87e90a6b806c78c3f180eefb001fb19d7d6a8) fixes it, so this MR pins it for now.